Stock-Based Compensation - Unvested restricted stock and deferred shares activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	$ 14.8
Total weighted average remaining term of unvested awards	1 year 6 months 26 days
Compensation cost recognized	$ 13.9	$ 13.4	$ 10.9
Performance stock units
Summary of unvested restricted stock and deferred shares activity and related information
Unvested at beginning of year (in shares)	267	201	107
Granted (in shares)	98	107	106
Cancelled/Forfeitures (in shares)	(38)	(41)	(12)
Vested (in shares)	(54)
Unvested at end of year (in shares)	273	267	201
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 56.96	$ 57.98	$ 56.97
Granted (in dollars per share)	60.45	55.27	58.94
Cancelled/Forfeitures (in dollars per share)	57.12	57.56	57.51
Vested (in dollars per share)	56.81
Unvested at end of period (in dollars per share)	$ 58.23	$ 56.96	$ 57.98
Other Disclosures
Total fair value of shares vested	$ 3.5	$ 0.0	$ 0.0
Total unrecognized compensation cost related to the unvested awards	$ 5.3
Total weighted average remaining term of unvested awards	1 year 6 months 4 days
Compensation cost recognized	$ 4.8	$ 4.0	$ 1.7
Aggregate intrinsic value of outstanding awards	$ 20.7
Performance stock units | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 75.95
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares
Summary of unvested restricted stock and deferred shares activity and related information
Unvested at beginning of year (in shares)	210	244	214
Granted (in shares)	139	140	180
Cancelled/Forfeitures (in shares)	(9)	(42)	(28)
Vested (in shares)	(123)	(132)	(122)
Unvested at end of year (in shares)	217	210	244
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 53.79	$ 52.61	$ 53.74
Granted (in dollars per share)	60.88	56.33	50.87
Cancelled/Forfeitures (in dollars per share)	55.55	54.43	53.99
Vested (in dollars per share)	55.35	53.10	51.72
Unvested at end of period (in dollars per share)	$ 57.31	$ 53.79	$ 52.61
Other Disclosures
Total fair value of shares vested	$ 7.7	$ 7.9	$ 6.6
Total unrecognized compensation cost related to the unvested awards	$ 8.3
Total weighted average remaining term of unvested awards	1 year 7 months 17 days
Compensation cost recognized	$ 6.9	$ 7.6	$ 6.7
Aggregate intrinsic value of outstanding awards	$ 16.5
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 75.95
Management Stock Purchase Plan | Restricted stock | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 75.95
Management Stock Purchase Plan | Restricted stock units (RSUs)
Summary of unvested restricted stock and deferred shares activity and related information
Granted (in shares)	47	89	60
Cancelled/Forfeitures (in shares)	(3)	(28)	(9)
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 16.84	$ 18.15	$ 19.04
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	$ 1.2
Total weighted average remaining term of unvested awards	1 year 4 months 24 days
Compensation cost recognized	$ 1.0	$ 0.7	$ 0.6
Aggregate intrinsic value of outstanding awards	6.3
Dividend declared and unpaid	$ 0.1